Magnetic-period mass spectrometer



May 31, 1955 1. e. SMITH 2,709,750

MAGNETIC-PERIOD MASS ISPECTROMETER Filed Oct. 1", 1951 2 Sheets-Sheet 1 ISnventor LING'OLN 6-. SMITH attorney May 31, 1955 L. s. SMITH MAGNETIC-PERIOD MASS SPECTROMETER Filed Oct. 17, 1951 2 Sheets-Sheet 2 V Ihmentor LINooLN 6. J'MITH flMflfiM Gttorncg on the ion.

MAGNETIC-PERIOD MASS SPECTRGMETER Lincoln G. Smith, Center Moriches, N. Y., assignor to the United States of America .as represented by the United States Atomic Energy Commission Application October '17, 19'5-1,'Ser ial No. 251,737

4 Claims. (Cl. 250-413) of deflection of the path of the charged particle caused by its passage through the field. More recently, mass spectrometers utilizing the perfect time and astigmatic space focusing of ions afterone or more complete revolutions in a-homogeneous magnetic field have been devised. it is known that an ion released in a magnetic field, in a perpendicular direction thereto, will describe a circular path in the field and return to its release point. The time for a complete revolution is dependent on the strength of the magnetic field, the mass of the ion and the charge Therefore, by=measuring the time for the ion to make a complete revolution in the field it is possible to determine the mass of the ion. This principle has been utilized by Bleakney and Hipple (Physical Review, volume 53, p. 521 (1938)) who employed a homogeneous magnetic field in conjunction with a perpendicular uniform electrostatic field. The function of the electrostatic field was to cause the centers of the circular-orbits of the ions to move perpendicularly to both the magnetic and electric fields. This permitted the ions to make more than one revolution in the magnetic field. However, a serious disadvantage oftn'is type of instrument is that the magnetic field and the electric field must be very uniform over a large area transverse to the magnetic field.

Another instrument, proposed by S. A. Goudsrnit and disclosed in U. S. patcnt application Serial Number 83,258, filed March 24, 1949, now 'Patent'No. 2,698,905, utilizes the same focal properties but releases the ions in the magnetic field so that they'will'follow a-helical path from the ion source to the detector. Again, this permits the ions to make more thanone complete revolution in the magnetic field but the number of revolutions of the ions is limited by the longitudinal extent-of the homogeneous magnetic field.

The present invention permits the ions to make many complete revolutions in the magnetic field without causing the centers of their orbits 'to continuously move transversely as in the Bleakney and Hippie instrument or axially as in the Goudsrnit apparatus. The present invention uses a uniform magnetic field in conjunction -with a pulsed electric field. Also, theelectric field employed is of small extent in time and space, is perpendicular to the magnetic field and is parallel to the direction of motion of the charged particles.

More particularly, the present invention relates to a mass spectrometer which includes an evacuated chamber placed in a homogeneous magnetic field. An ion source located in the chamber emits the charged particles in a direction perpendicular to the magnetic field. The particles follow a circular orbit of motion for 180 whereupon a voltage pulseis applied across theparticles parallel to their direction of -motion and perpendicular to the magnetic field decelerating the :particles and .causing'them nitcd States Patent side wall 32 of the detector.

Patented May 31, 1955 to follow a shorter orbit. The decelerated particles are allowed to revolve a predetermined integral number of times and then they are subjected to a second voltage pulse causing further deceleration and prompt impingement upon a detector located in the magnetic field.

It is accordingly one of the objects of the present invention to provide a new and improved apparatus for measuring the mass of charged particles.

A second object of the invention is to provide a new and improved mass spectrometer using a magnetic field and a pulsed electric field.

A further object of the invention is to provide a new and improved mass spectrometer using a small transverse area of magnetic field and a perpendicular electric field.

Another object of the invention is to provide a new and improved magnetic period mass spectrometer wherein the centers of the orbits of motion of the ions do not continuously move axially or transversely in the magnetic field.

It is also an object of the present invention to provide a mass spectrometer for accurately measuring the packing fractions of various isotopes.

The many objects and advantages of the present invention may best be appreciated by reference to the accompanying drawings, the figures of whichillustrate apparatus incorporating a preferred embodimentof the present invention and capable of carrying out the method of the invention. In the drawings:

Figure l is a perspective view of the apparatus with the top cover removed to show the interior constructional details.

Figure 2 is a schematic representation of the apparatus shown in conjunction with conventional electronic instruments.

Figure 3 shows a typical response pattern of the apparatus.

Referring to Figure l the apparatus is'mounted within a chamber adapted to be evacuated comprising a circular wall 10 and two cover plates, only one of which, 11, is shown. An ion source is located near the inside surface of the wall 19. Connections to the ion source are made through the wall of the vacuum chamber bymeans of a conventional connector 21. Extending from the center of connector 21 is a tube 22 through which a sample ofthe gas to be measured may be introduced into the ion source. Connector 21 also includes conductors for supplying electrical operating potentials to the interior of the ion source 20. The source 20 may be any conventional ion source, such as the electron bombardment type described on page 231 of volume I of Advances in Electronics, edited by L. Marten, First edition, published by the Academic Press Inc. The ions whose masses are to bemeasured are emitted from the source through the exit slit 23. Next to the ion source and spaced radially inwardly in the interior of'the chamber is a detector for-charged particles. Any detector capable of operating in a magnetic field, such as the detector described in my copending United States patent application S. N. 232,893(48), filed June 22, 1951, entitled Magnetic Electron Multiplier, now Patent No. 2,664,515, or the detector developed by Richards and Hays, and disclosed in copending United States patent application S. N. 199,294 (48), filed December 5, 1950, may be used. The detector described in my above-mentioned copending application is shown in Figure l with the top wall partially broken away to expose the entrance window 31 in the Diametrically disposed to the ion source 20 and mounted on the inside of Wall 10 of the vacuum chamber is the pulsing electrode with adjacent grounded electrodes 41 and 42. Electrodes 40,

Connection to the center electrode 40 is made by a conventional connector 47 through the wall of the vacuum chamber. Provided in the center of the connector 47 is a coaxial cable 4-8. The slit 43 of electrode 40, window 31 of the detector 30 and exit slit 23 are all aligned along a diameter of the vacuum chamber. Halfway between the ion source and the pulsing electrode 40 on the wall 10 is mounted a conventional connector 49 for evacuation purposes. The remaining connector 51 contains high voltage conductors for supplying operating potentials to the detector 3%? as well as an output lead to receive the amplified output of the detector. Connections between connector 51 and detector 30 are made through a shielded trough 52 in the cover 11 of the vacuum chamber. Also included within the vacuum chamber and spaced a small distance from the wall 10 is an electrostatic shield 53 to prevent any of the outside connections from interfering with the operation of the device. To permit passage of the charged particles through the shield 53, a slit 54, aligned with exit slit 23 of source 20, is included.

Referring now to Figure 2 the operation of the embodiment described in Figure 1 will be explained in detail. In Figure 2 is shown the operating components of the apparatus in conjunction with conventional electronic instruments. Connected to the ion source is a power supply and pulse generator titl. Connected to the detector is a high voltage supply "in and an amplifier Sll which is connected to an oscilloscope 90. To the pulsing electrode 40 is connected a pulse generator 1%. In operation the evacuated chamber is placed between the poles of a magnet so that the induced magnetic field is perpendicular to the plane of Figure 2. The gas containing the isotopes whose masses are to be measured is introduced into the ion source 20 through tube 22 shown in Figure l. A positive pulse of voltage is applied to the electron accelerating electrodes of the source 20 from supply as, ionizing the gas and causing charged particles of the gas to be emitted from window 23 into the magnetic field in a perpendicular direction to the lines of fiux. These ions are deflected by the magnetic field and follow a circular orbit of motion indicated by the dotted line 12 and arrow 13. The particles are permitted to remain solely under the influence of the magnetic field for 180 at which time they are passing through the slit 43 in the pulsing electrode 4t). At this time a negative rectangular voltage pulse is applied to the pulsing electrode 49 by means of the pulse generator 160 and the cable 418. This negative pulse establishes an electric field between electrodes 4t) and 42 parallel to the direction of motion of the particles. Since the ions are positively charged, any ions between slits 43 and 45 will be attracted towards the more negative electrode 40 thereby causing their velocities to be reduced. Those particles whose momenta have been reduced the proper amount follow a shorter orbit of motion indicated by the dotted line 14 and arrow l5. These decelerated particles therefore miss the ion source 20 and continue around in their new circular orbit. If these particles were permitted to continue indefinitely they would be dispersed due to scattering caused by the residual gas still in the vacuum chamber. instead, after a predetermined integral number of revolutions from the pulsing electrode 40, a second negative rectangular voltage pulse is applied to this electrode from pulse generator 1%. This establishes an electric field which further decelerates the particles between slits/l3 and 4S and causes them to follow the still shorter orbit indicated by dotted line 16 and arrow 17. After passing through 180 of their new orbit, the ions enter detector 3% through the window 31 where they are amplified and the resulting output pulse is applied to the amplifier 80 and exhibited on oscilloscope 90. The time between the first and second pulses supplied by generator liitl is a measure of the period of rotation of the particles in the magnetic field. The mass of the particles can therefore be determined from the Well known equation T=21r(mc/eH), where T is the period of rotation of the ion in the magnetic field; m is the mass of the ion; 0 equals the velocity of light; e equals the magnitude of the charge carried by each ion and H is the magnitude of the magnetic field. The polarity, amplitude and width of the first and second pulses emitted by generator are equal so that the orbit of the decelerated particles are changed very nearly equal amounts by each pulse. Therefore the distance 18, between the exit slit 23 of source 20 to the dotted line 14 representing the second orbit of motion, is very nearly equal to the distance 19, between the dotted line 14 and the entrance window 31 of detector 30.

The rotation period of the charged particles is determined from the time taken by a maximum number of the particles leaving exit slit 23 to arrive at entrance window 31 of the detector. Therefore, when measuring this rotation period, the time interval between the first and second pulse emitted by generator 100 is slightly varied until a maximum peak has been indicated on the oscilloscope 90. This will be more fully explained with reference to a specific example hereinbelow.

The apparatus described above is particularly well adapted for the measurement of packing fractions or mass defects of various isotopes to determine the exact masses of the isotopes. The mass defect is the difference between the actual value of the mass of an isotope and the nearest integral mass. These mass defects are characterized by numbers called packing fractions which are defined by the equation PF X 10 where I is the nearest integer to M and M is the exact mass of the isotope under investigation in units of onesixteenth of the lightest oxygen isotope which is defined to be mass 16.0000.

For example, to determine the exact mass of sulfur-32, sulfur dioxide is admitted into the ion source 20. The source 20 forms ion bunches of S+ and 02+ which are emitted into the magnetic field through exit slit 23. Since the approximate masses of the charged particles are known, the time for these particles to traverse of their orbit can be ascertained and generator 100 is operated to emit its first negative pulse at this time, decelerating the S and 02+ charged particles. These particles are permitted to rotate approximately 40 times in their shorter orbit before the second voltage pulse is applied and the resulting output peak exhibited on oscilloscope 90. The time interval between the first and second generated pulse is varied over a small range corresponding to the approximation of the rotation period and that time interval which results in the maximum amplitude of the detected peak exhibited on oscilloscope 90 is equal to 40 rotational periods of the ions. During the actual measurement performed with this apparatus the magnetic field was 850 oersteds and the pressure was appoximately l0 mm. Hg. The optimum time interval between the first and second generated pulses equalled 840 microseconds. Referring to Figure 3 the peaks exhibited on the oscilloscope 90 are shown. Actually only one of the peaks is seen on the oscilloscope screen at any one time. That is, as the time interval between the first and the second generated pulses is increased, a peak representing the arrival of the S ions will be seen on the oscilloscope screen. The height of this peak is measured on the screen and the time interval between the generated voltage pulses is slowly varied until a maximum S peak is obtained. This time interval is equal to 40 rotational periods of the S ions. Since the 02 ions are slightly heavier they will take slightly longer to traverse 40 of their orbits. Therefore the time interval between the first and second generated pulses is increased until the S peak no longer is exhibited on the oscilloscope screen and further increased until the 02 peak is seen. The height of this peak is measured and the time interval slowly varied until the maximum height is obtained. This maximum height of the 02 peak indicates that a maximum number of the 02 ions emitted from the ion source 20 are reaching the detector entrance window 31. The difierence between the time interval necessary to obtain a maximum S peak and the time interval necessary to obtain a maximum 02 peak, represented by t in Figure 3, is a measure of the mass difierence between 02 and 8. Since oxygen has been assigned an integral mass 16, 02 will have an exact mass 32 and the time interval, t, determines the exact diifer'ence between integral mass 32'of oxygen and the mass of sulfur-32. This time measurement may easily be made with a conventional time delay circuit. In this manner a value of S =31.9823i0.00l0 was obtained.

In the specific example described above a relative measurement of the mass of S was made. That is, the mass of sulfur-32 was measured in terms of the mass of 02. This is generally the most accurate means of mass measurement as both the S and the 02 have been simultaneously subjected to the magnetic and electric fields. Therefore, any inhomogeneity in the field or other changes which may occur during the measuring interval will aifect both in very nearly the same manner. In using the method of relative mass measurement it is not necessary to determine the total time interval between the first and second generated voltage pulses with greater accuracy than the time interval between the two detected peaks. This results from the fact that the following equation can be used for relative mass measurements: AM/M=t/ T where M is the known integral mass; AM is the difference in mass between M and the element under measurement; t is the difference in time between the rotational periods of the two elements in the magnetic field, i. e. the time between the two detected peaks; and T is the time between the first and second generated pulses. However, the instrument can be used for the absolute measurement of isotope masses by allowing the ions of the isotope alone to be deflected and to revolve in the manner hereinabove indicated. In this instance the total time between the two generated voltage pulses would have to be measured with considerably greater precision.

The electronic instruments used with the device are wholly conventional. The pulse generator 100 may include a thyratron for establishing the negative voltage pulses. The firing of the thyratron could be accomplished by standard blocking tube oscillator circuits or the like. The time intervals between the application of the first and second pulses may easily be varied with standard time delay circuits. One oscilloscope that was found to be satisfactory for operation was the oscilloscope type 256-D manufactured by the A. B. Du Mont Laboratories, Inc,, Clifton, New Jersey. This oscilloscope also includes an internal trigger generator with a variable time delay which can be used for triggering the pulse generator 100. Actually, it is not necessary to use an oscilloscope, as any meter that will indicate the amount of ions arriving at the detector can be used. The high voltage supply 70 should be capable of delivering voltages of the order of 5000 volts. To illustrate the small volume required for a spectrometer constructed in accordance with Figure 1, an embodiment which operated satisfactorily was placed in the 2 inch gap of an electromagnet with poles 15 inches in diameter. The orbit illustrated by dotted line 12 in Figure 2 was approximately 11 inches in diameter while the shorter orbits were and 9 inches respectively. With this apparatus ions of masses 18, 28, and 44 were detected after 70, 40, and 25 rotations between pairs of pulses.

The operation of the apparatus described with respect to Figures 1 and 2 may be further improved by the addition of standard electrostatic or magnetic focusing lenses to reduce spreading of the beam along the magnetic field and thus result in a more intense detected pulse. The vacuum chamber can be made of an electrically conductive material so that it can be grounded and serve as an electrostatic shield. Electrodes 41 and 42 are then connected to the electrical ground by being mounted on the wall 10 of the chamber. The pulser electrode 40 can be electrically insulated from ground so that a difference of potential can exist between it and electrodes 41 and 42. It is apparent that the entrance window 31 of the detector may be spaced a greater distance from the exit slit 23 of the ion source so that the particles to be measured may be submitted to the electric field more than twice before being detected. The particles would then be detected after transversing three or more orbits of motion. This same result can be accomplished by decreasing the amplitude of the negative rectangular voltage pulse applied to the pulser electrode 40 so that the particles may actually follow more than two new orbits before reaching the entrance window of the detector. The apparatus will also operate properly if the detector is placed radially outward with respect to the ion source. Pulse generator would then supply a positive rectangular voltage pulse to pulsing electrode 40 and accelerate the particles thereby increasing the diameter of the new orbit so that the particles will again miss the ion source.

While the salient features of this invention have been described in detail with respect to one embodiment it will of course be apparent that numerous modifications may be made within the spirit and scope of this invention and it is therefore not desired to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

I claim:

1. Apparatus for measuring ion masses which comprises in combination, means for establishing a homogeneous magnetic field, a chamber arranged to be evacuated located in said field, an ion source located in said chamher, said source being arranged for the emission of positive ions in a direction perpendicular to the lines of flux of said magnetic field whereby said ions are deflected into a circular orbit of motion, means for subjecting said ions to electric fields of short time duration transverse to said magnetic field and parallel to the direction of motion of said ions whereby the orbit of motion of a discrete group of said ions is decreased each time said discrete group of ions is subjected to the electric field and means for detecting said discrete group of ions in a decreased orbit.

2. Apparatus for measuring ion masses which comprises in combination, means for establishing a homogeneous magnetic field, a chamber arranged to be evacuated located in said field, an ion source adjacent the periphery of said chamber, said source being arranged for the emission of ions in a direction perpendicular to the lines of flux of said magnetic field whereby the ions are deflected into a circular orbit of motion, means for intermittently establishing an electric field of short time duration transverse to said magnetic field and parallel to the direction of motion of said ions, said electric field establishing means being diametrically disposed in said chamber with respect to said ion source whereby the orbit of motion of a discrete group of said ions is decreased each time said discrete group of ions is subjected to the electric field and means for detecting said discrete group of ions in a decreased orbit.

3. Apparatus for measuring the masses of charged particles which comprises in combination, means for establishing a homogeneous magnetic field, a chamber arranged to be evacuated located in said field, an ion source adjacent the periphery of said chamber, said source being arranged for the emission of said charged particles in a direction perpendicular to the lines of flux of the said magnetic field whereby the particles are deflected into a circular orbit of motion, an electrode for establishing an electric field transverse to said magnetic field and parallel to the direction of motion of said particles, said electrode being diametrically disposed in said chamber with respect to said ion source, pulse generating means connected to said electrode for intermittently energizing the electrode for brief time intervals whereby the orbit of motion of a discrete group of said charged particles is decreased each time said discrete group of charged particles is subjected to the electric field and means for detecting said particles in said decreased orbit of motion.

4. Apparatus for measuring the masses of charged particles which comprises in combination, means for establishing a homogeneous magnetic field, an electrostatically shielded grounded chamber arranged to be evacuated located in said field, an ion source adjacent the periphery of said chamber, said source being arranged for the emission of said charged particles in a direction perpendicular to the lines of flux of said magnetic field whereby the particles are deflected into a circular orbit of motion, an electrode electrically insulated from said chamber and diametrically disposed in said chamber with respect to said ion source, two grounded electrodes adjacent said insulated electrode and on opposite sides thereof, each of the three electrodes including a slit to permit the passage of the charged particles therethrough, pulse generating means connected to said insulated electrode for intermittently applying a rectangular voltage pulse to said insulated electrode thereby establishing a decelerating electric field transverse to said magnetic field and parallel to the direction of motion of said particles whereby the orbit of motion of a discrete group of said particles may be decreased each time the discrete group of particles is subjected to said electric field and means for detecting said particles in said decreased orbit of motion.

References Cited in the file of this patent UNITED STATES PATENTS 

1. APPARATUS FOR MEASURING ION MASSES WHICH COMPRISES IN COMBINATION, MEANS FOR ESTABLISHING A HOMOGENEOUS MAGNETIC FIELD, A CHAMBER ARRANGED TO BE EVACUATED LOCATED IN SAID FIELD, AN ION SOURCE LOCATED IN SAID CHAMBER, SAID SOURCE BEING ARRANGED FOR THE EMISSION OF POSITIVE IONS IN A DIRECTION PERPENDICULAR TO THE LINES OF FLUX OF SAID MAGNETIC FIELD WHEREBY SAID IONS ARE DEFLECTED INTO A CIRCULAR ORBIT OF MOTION, MEANS FOR SUBJECTING SAID IONS TO ELECTRIC FIELDS OF SHORT TIME DURATION TRANSVERSE TO SAID MAGNETIC FIELD AND PARALLEL TO THE DIRECTION OF MOTION OF SAID IONS WHEREBY THE ORBIT OF MOTION OF A DISCRETE GROUP 